Designing Your Incident Severities
incident.io
incident.io
Lots of the literature around this is dense and intended for mega-corps, not your average growing company.
If you're ever unlucky enough to be building your company's incident response process, this is pragmatic "just do this and it'll be fine" advice, based on the experience of many companies doing a similar thing.
Edit: I think this was based on observation (from a safe distance) of a complete set of managers right up to the CIO standing behind the desk of someone in the helpdesk office.